Field engineers in oil and gas drilling operations can make costly errors if the viscosity specifications of fluids pumped downhole are incorrect. To reduce such errors, viscosity tests that simulate downhole conditions are often performed at the surface to ascertain the behavior of fluids at different temperatures and pressures before they are pumped downhole.
There are different types of viscosity testing techniques. One example technique involves a rotational rheometer tool that shears a test fluid between two surfaces. The speed at which an object in contact the test fluid rotates in response to a constant torque can be correlated with fluid viscosity. Alternatively, the torque required to rotate an object in contact with the test fluid at a constant speed can be correlated with fluid viscosity.
While the above viscosity testing concepts are straightforward, building a tool that accurately measures torque or rotation speed is problematic. Previous efforts to measure torque include attaching a strain sensor (e.g., a spring) to a shaft, where a bob attached to the shaft is at least partially immersed in the test fluid. In this configuration, the strain sensor tracks the strain applied to the shaft as the test fluid interacts with the bob due to rotation of a housing or “rotor” that encloses the fluid and bob. Unfortunately, strain sensor accuracy often degrades over time. Further, the strain at the top of the shaft where the strain sensor is located is not an accurate reading of the strain at the bottom of the shaft where the bob is located. This is especially problematic if the test involves elevated temperatures or pressures, which require a sealed enclosure (i.e., the seals needed to maintain elevated temperatures or pressures contact the shaft and interfere with strain sensor accuracy).
It should be understood, however, that the specific embodiments given in the drawings and detailed description do not limit the disclosure. On the contrary, they provide the foundation for one of ordinary skill to discern the alternative forms, equivalents, and modifications that are encompassed together with one or more of the given embodiments in the scope of the appended claims.